Surfactants are well known materials which can be generally described as having a hydrophobic moiety and a hydrophilic group per molecule. A wide variety of these materials are known and are classified as anionic, cationic, nonionic and amphoteric. They are well known to have numerous uses such as emulsifiers, detergents, dispersants and solubilizing agents in the field of cosmetics, textile treatment, industrial and personal cleaning preparations, corrosion inhibitors and the like.
Emulsification in particular is an extremely important technology and property of surfactants and it is a process which is used in detergency, emulsion polymerization, cosmetics, food, agrochemicals, paints, paper, transportation of crude oil, etc. Emulsifiers function as essential ingredients in personal care and household products; industrial and institutional cleaners including hair shampoos, car wash detergents, carpet shampoos, hand dishwashing liquids, latex foaming and oil recovery compositions; and the like.
In order to form a relatively stable emulsion, an emulsifier is required to adsorb at an oil-water interface to prevent emulsion droplet coalescence. The majority of emulsifiers are synthetic surfactants or natural products with amphiphilic characteristics. Presently, usage levels of surfactants that are necessary for effective emulsification are usually above 0.1% active based on the total weight of the detergent solution used in the final composition. For a given emulsifier system, it would obviously be advantageous to be able to use a lower amount of surfactant in order to reduce the cost of the surfactant and amount of surfactant waste discharged into the environment, while at the same time improving the performance of the final products e.g., improving film forming and water resistance in latex paints and reducing skin irritation in cosmetic products!.
Surfactants generally are compounds having one hydrophilic group and one hydrophobic moiety. Recently, a group of compounds having two hydrophobic moieties and two hydrophilic groups have been introduced. These have become known as "gemini surfactants" in the literature (Chemtech, March 1993, pp 30-33), and J. Am. Chem. Soc., 115, 10083-10090, (1993) and the references cited therein. Since their introduction, cationic and anionic "gemini surfactants" have been disclosed. Other surfactant compounds having two hydrophilic groups and two hydrophobic moieties have been disclosed but not referred to as gemini surfactants.
It has been found that gemini surfactants are unexpectedly effective emulsifiers when used at very low concentrations in comparison to conventional surfactants. This finding is reflected in their ability to afford superior detergency at very low concentrations. Other performance properties related to emulsification as mentioned above can be also improved using low concentrations of gemini surfactants.
The novel surfactant compounds of the invention are based on certain gemini surfactants. As used herein, the term "gemini surfactants" is intended to mean surfactants having at least 2 hydrophobic groups and at least 2 hydrophilic groups per molecule.
A number of the gemini surfactants are reported in the literature, see for example, Okahara et al., J. Japan Oil Chem. Soc. 746 (1989); Zhu et al., 67 JAOCS 7,459 (July 1990); Zhu et al., 68 JAOCS 7,539 (1991); Menger et al., J. Am. Chem. Soc. 113, 1451 (1991); Masuyama et al., 41 J. Japan Chem. Soc. 4,301 (1992); Zhu et al., 69 JAOCS 1,30 (January 1992); Zhu et al., 69 JAOCS 7,626 (July 1992); Menger et al., 115 J. Am. Chem. Soc. 2, 10083 (1993); Rosen, Chemtech 30 (March 1993); and Gao et al., 71 JAOCS 7,771 (July 1994), all of this literature being incorporated herein by reference.
Also, gemini surfactants are disclosed by U.S. Pat. No. 2,374,354 to Kaplan; U.S. Pat. Nos. 2,524,218 and 2,530,147 to Bersworth (two hydrophobic tails and three hydrophilic heads); U.S. Pat. No. 3,244,724 to Guttmann; and U.S. Pat. No. 5,160,450, to Okahara, et al., all of which are also incorporated herein by reference.
The gemini surfactants may also be anionic, nonionic, cationic or amphoteric. The hydrophilic and hydrophobic groups of each surfactant moiety may be any of those known to be used in conventional surfactants having one hydrophilic group and one hydrophobic group. For example, a typical nonionic gemini surfactant, e.g., a bis-polyoxyethylene alkyl ether, would contain two polyoxyethylene alkyl ether moieties. However, non-ionic gemini surfactants prior to this are extremely rare. Anionic gemini surfactants carry a negative charge on the hydrophilic portion, usually in the form of a carboxylate, phosphate, sulfate or sulfonate.
Gemini surfactants are relatively quite new and few species have been reported or disclosed in the prior art. U.S. Pat. No. 5,534,197 to Scheibel teaches a method for the preparation of a nonionic gemini surfactant wherein the hydrophilic head is a sugar or carbohydrate while the hydrophobic head is a long chain alkyl, the two being joined by a short alkyl chain. U.S. Pat. Nos. 3,888,797 and 3,855,156, both to Marumo, disclose a number of nonionic gemini surfactant species in which the hydrophobic portion is comprised of a long chain lower alkyl or alkene while the hydrophilic portion is comprised of an ethoxylate group. U.S. Pat. No. 4,892,806 to Briggs and European Patent No. 0,688,781A1 to Adams also disclose sugar-based hydrophilic heads joined to the hydrophobic counterpart by a short chain carbon bridge.
Sulfate, phosphate, and carboxylate surfactants are currently disclosed in the literature (See JAOCS 67, 459 (1990); JAOCS 68, 268 (1991); JAOCS 68, 539 (1991); and JAOCS 69, 626 (1992). In each case, a secondary hydroxyl is sulfated, carboxylated, or phosphated.
Secondary hydroxyl's phosphate poorly in that phosphoric anhydride leads to olefin production (dehydration) while polyphosphoric acid results in high levels of phosphoric acid and unphosphated material. The present invention results in compounds which are characterized by primary hydroxyl groups which can more efficiently be functionalized.
Similarly, sulfation can also lead to dehydration by-products. Carboxymethylation of secondary hydroxyl groups is also difficult resulting in low yields.
One author J. Am. Chem. Soc. 115, 10,083 (1993) and J. Am. Chem. Soc. 113, 1451 (1991)! prepares a phosphate moiety on a primary hydroxyl group. But in these references, it is necessary to utilize mixed alcohols in order to incorporate a hydrophobe into the molecule. This leads to the production of mixed diphosphates, a necessary outgrowth of using the mixed alcohols. This difficulty is eliminated in the present invention. In addition, high monoalkylphosphates as well as diphosphates can be prepared according to the present invention.
Due to the need for new and more effective and efficient surfactants, as well as the need for mild surfactants which are biologically compatible in an ecologically sensitive environment, effort has been made to develop a new class of compounds which demonstrate improved surface-active properties that are further characterized as mild and environmentally benign.